Thermal Expansion
Determine if a check valve exists in the inlet water line. Check with your local water utility
company. It may have been installed in the cold water line as a separate back flow preventer, or
it may be part of a pressure reducing valve, water meter or water softener. A check valve located
in the cold water inlet line can cause what is referred to as a “closed water system”. A cold water
inlet line with no check valve or back flow prevention device is referred to as an “open” water
As water is heated, it expands in volume and creates an increase in the pressure within the water
system. This action is referred to as “thermal expansion”. In an “open” water system, expanding
water which exceeds the capacity of the water heater flows back into the city main where the
pressure is easily dissipated.
A “closed water system”, however, prevents the expanding water from flowing back into the
main supply line, and the result of “thermal expansion” can create a rapid and dangerous
pressure increase in the water heater and system piping. This rapid pressure increase can quickly
reach the safety setting of the relief valve, causing it to operate during each heating cycle. Thermal
expansion, and the resulting rapid, and repeated expansion and contraction of components in the
water heater and piping system can cause premature failure of the relief valve, and possibly the
heater itself. Replacing the relief valve will not correct the problem!
The suggested method of controlling thermal expansion is to install an expansion tank in the cold
water line between the water heater and the check valve (see illustration below). The expansion
tank is designed with an air cushion built in that compresses as the system pressure increases,
thereby relieving the over pressure condition and eliminating the repeated operation of the relief
valve. Other methods of controlling thermal expansion are also available. Contact your installing
contractor, water supplier or plumbing inspector for additional information regarding this subject.
Typical Installation
Refer to the illustration below for suggested typical installation. The installation of unions or
flexible copper connectors is recommended on the hot and cold water connections so that the
water heater may be easily disconnected for servicing if necessary. The HOT and COLD water
connections are clearly marked and are 3/4” NPT on all models. Install a shut-off valve in the cold
water line near the water heater.
Water Supply Connections
apply heat to the HOT or
COLD water connections. If
sweat connections are used,
sweat tubing to adapter
before fitting adapter to the
cold water connections on
heater. Any heat applied
to the cold water supply
fittings will permanently
damage the dip tube.
NOTICE: The National
Fuel Gas Code (NFGC)
mandates a manual
gas shut-off valve: See
(NFGC) for complete
instructions. Local codes
or plumbing authority
requirements may vary
from the instructions or
diagrams provided and
take precedent over these
Vacuum Relief Valve
(Not Supplied)
If required, install per local codes
and valve manufacturer’s
Heat trap
6” minimum
Heat trap
6” minimum
To gas supply
Sediment trap
Ground joint union
Drain Pan Pipe
to suitable drain.
Relief valve discharge
line to suitable open
To cold water
6” Air gap
Draft hood
Pitch up 1/4”
per foot
Auxiliary catch pan
Manual gas shut-off
Thermostatic gas valve
Jacket door
Temperature and
pressure relief valve
Shut-off valve
Thermal expansion
tank (if required)
Combustion Air
Inlet Openings
Hot water outlet to
Water Heater Jacket
Vent connector to
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